God save the Queen — with a hand from a former Harlem high-school student!

Native New Yorker Denton John, 23, will be the only American working the wedding of Prince William and Kate Middleton — as a proud member of the Household Cavalry Mounted Regiment, an elite branch of the British military charged with protecting the royals, especially Queen Elizabeth.

Sporting an elaborate uniform which includes a red jacket, silver breast plate and helmet and a sword, John — who grew up on the Upper West Side — will ride on horseback in the procession escorting The Queen when she leaves Westminster Abbey after the nuptials.

Not bad for a city kid whose “riding” experience was mostly on the subway.

“I was always sure he was going to do something positive with his life,” said Loretta Lewis, his former teacher at Wadleigh HS on West 114th Street.

“He would come to school in a tie every day, always cool — and this was before mandatory uniforms!” she gushed.

John’s love of all things British bloomed at the school, where he studied the English classics like “Beowulf” and “Pride and Prejudice.”

As a dual citizen of the US and Grenada — part of the British Commonwealth — John is allowed to serve in the British military.

He says the honor has thrilled his mother: “My mom’s very proud. She’s telling everyone about this,” he told CNN International.